U.S. patent number 5,172,343 [Application Number 07/802,804] was granted by the patent office on 1992-12-15 for aberration correction using beam data from a phased array ultrasonic scanner.
This patent grant is currently assigned to General Electric Company. Invention is credited to Matthew O'Donnell.
United States Patent |
5,172,343 |
O'Donnell |
December 15, 1992 |
Aberration correction using beam data from a phased array
ultrasonic scanner
Abstract
A PASS ultrasonic system performs a scan in which phase errors
due to aberrations in the sound media are corrected prior to the
acquisition of each beam. Phase errors are measured by cross
correlating each of a set of reference beams with the desired beam
to produce beam forming errors as a function of beam angle. This
function is Fourier transformed to produce phase corrections that
are employed by the ultrasonic system to offset the phase
errors.
Inventors: |
O'Donnell; Matthew (Ann Arbor,
MI) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
25184748 |
Appl.
No.: |
07/802,804 |
Filed: |
December 6, 1991 |
Current U.S.
Class: |
367/7; 367/103;
367/105; 367/11; 600/447; 73/626 |
Current CPC
Class: |
G01N
29/0645 (20130101); G01N 29/30 (20130101); G01S
7/52047 (20130101); G01S 7/52049 (20130101); G01S
7/52095 (20130101); G10K 11/345 (20130101) |
Current International
Class: |
G01N
29/22 (20060101); G01N 29/30 (20060101); G01S
7/52 (20060101); G10K 11/34 (20060101); G10K
11/00 (20060101); G03B 042/06 () |
Field of
Search: |
;367/7,11,103,105
;73/626 ;128/661.01 ;364/413.25,413.13 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pihulic; Daniel T.
Attorney, Agent or Firm: Snyder; Marvin
Claims
What is claimed is:
1. A coherent vibratory energy beam imaging system which corrects
for aberrations in transmission media for said beam,
comprising:
a transducer array having a set of array elements disposed in a
pattern and each being separately operable to produce a pulse of
vibratory energy during a transmission mode and to produce an echo
signal in response to vibratory energy impinging thereon during a
receive mode;
a transmitter coupled to said transducer array and being operable
during the transmission mode to apply a separate signal pulse to
each array element such that a steered transmit beam is
produced;
a receiver coupled to said transducer array and being operable
during the receive mode to sample the echo signal produced by each
array element as the vibratory energy impinges thereon and to form
a plurality of simultaneous receive beam signals therefrom by
summing the separate echo signals sampled from each array
element;
controller means coupled to said transmitter and said receiver and
being operable to perform a scan during which a set of correlation
measurements is made periodically to measure phase of the echo
signals steered in a set of reference beam angles with respect to
the echo signal steered in a desired beam angle (.theta.); and
processor means coupled to said transmitter, said receiver and said
controller means and being operable in response to the simultaneous
receive signals produced as a result of the periodic correlation
measurements to generate a set of errors (A.theta.,
.DELTA..phi..sub..theta.) and perform a Fourier transformation
thereon to produce a set of aberration phase corrections
.DELTA..phi..sub.k for application to the transmitter and the
receiver to correct the phase of the signals applied to and
produced by said each array element in the transducer array.
2. The coherent vibratory energy beam imaging system recited in
claim 1 wherein said controller means comprises means for cross
correlating a portion of the simultaneous receive beam signals.
3. The coherent vibratory energy beam imaging system recited in
claim 1 wherein said receiver includes means for producing a pair
of simultaneous receive beam signals from the echo signals
4. The coherent vibratory energy beam imaging system recited in
claim 1 wherein said processor means includes interpolation means
for generating aberration phase corrections for any beam angle.
5. In an ultrasonic imaging system, a method of correcting for
aberrations in transmission media for said beam, comprising:
energizing a set of ultrasonic transducer array elements in an
ultrasonic transducer array, said array elements being disposed in
a pattern and each being separately operable in response to a
separate signal pulse applied to each array element during a
transmission mode such that a steered transmit beam is produced,
said array elements further being separately operable to produce an
echo signal in response to ultrasonic energy impinging thereon
during a receive mode;
sampling the echo signal produced by each array element as the
ultrasonic energy impinges thereon;
forming a plurality of simultaneous receive beam signals from said
set of array elements by summing the separate echo signals sampled
from each array element;
performing a scan in which a set of correlation measurements is
made periodically during the scan to measure phase of the echo
signals steered in a set of reference beam angles with respect to
the echo signal steered in a desired beam angle (.theta.);
generating a set of errors (A.theta., .DELTA..phi..sub..theta.) in
response to the simultaneous receive signals produced as a result
of the periodic correlation measurements; and
Fourier transforming the set of errors (A.theta.,
.DELTA..phi..theta.) to produce a set of aberration phase
corrections .DELTA..phi..sub.k for correcting the phase of the
signals applied to and produced by each array element in the
ultrasonic transducer array.
6. The method recited in claim 5 wherein the set of correlation
measurements is made prior to acquiring each beam of sample data
employed to reconstruct an image and the aberration phase
corrections .DELTA..phi..sub.k are coupled to the transmitter and
receiver prior to the acquisition of each such beam of sample
data.
7. The method recited in claim 5 wherein each correlation
measurement is made by cross correlating a portion of the
simultaneous receive beam signals.
8. The method recited in claim 5 wherein a pair of simultaneous
receive beam signals is generated from the echo signals, and
wherein each correlation measurement is made by cross correlating a
portion of the beam sample data from said pair of simultaneous
receive beam signals.
9. The method of claim 8 wherein one of said correlation
measurement in each set thereof includes receive beam signals
steered in the desired beam angle (.theta.) and receive beam
signals steered in one of said reference beam angles, and wherein
the remaining correlation measurements in said each set thereof
includes receive beam signals steered in two of said reference beam
angles.
10. The method recited in claim 5 wherein each correlation
measurement is made from a set of receive beam signals emanating
from a random collection of sound scatters.
11. The method recited in claim 9 wherein the beam angle (.theta.)
transmitted for aberration correction processing corresponds in
direction to one of said reference beam angles.
Description
BACKGROUND OF THE INVENTION
This invention relates to coherent imaging systems using vibratory
energy, such as ultrasound, and, in particular, to ultrasound
imaging systems which employ phased array sector scanning.
There are a number of modes in which vibratory energy, such as
ultrasound, can be used to produce images of objects. The
ultrasound transmitter may be placed on one side of the object and
the sound transmitted through the object to the ultrasound receiver
placed on the other side ("transmission mode"). With transmission
mode methods, an image may be produced in which the brightness of
each pixel is a function of the amplitude of the ultrasound that
reaches the receiver ("attenuation" mode), or the brightness of
each pixel is a function of the time required for the sound to
reach the receiver ("time-of-flight" or "speed of sound" mode). In
the alternative, the receiver may be positioned on the same side of
the object as the transmitter and an image may be produced in which
the brightness of each pixel is a function of the amplitude or
time-of-flight of the ultrasound reflected from the object back to
the receiver ("refraction", "backscatter" or "echo" mode). The
present invention relates to a backscatter method for producing
ultrasound images.
There are a number of well-known backscatter methods for acquiring
ultrasound data. In the so-called "A-scan" method, an ultrasound
pulse is directed into the object by the transducer and the
amplitude of the reflected sound is recorded over a period of time.
The amplitude of the echo signal is proportional to the scattering
strength of the refractors in the object and the time delay is
proportional to the range of the refractors from the transducer. In
the so-called "B-scan" method, the transducer transmits a series of
ultrasonic pulses as it is scanned across the object along a single
axis of motion. The resulting echo signals are recorded as with the
A-scan method and either their amplitude or time delay is used to
modulate the brightness of pixels on a display. With the B-scan
method, enough data are acquired from which an image of the
refractors can be reconstructed.
In the so-called C-scan method, the transducer is scanned across a
plane above the object and only the echoes reflecting from the
focal depth of the transducer are recorded. The sweep of the
electron beam of a CRT display is synchronized to the scanning of
the transducer so that the x and y coordinates of the transducer
correspond to the x and y coordinates of the image.
Ultrasonic transducers for medical applications are constructed
from one or more piezoelectric elements sandwiched between a pair
of electrodes. Such piezoelectric elements are typically
constructed of lead zirconate titanate (PZT), polyvinylidene
difluoride (PVDF), or PZT ceramic/polymer composite. The electrodes
are connected to a voltage source, and when a voltage is applied,
the piezoelectric elements change in size at a frequency
corresponding to that of the applied voltage. When a voltage pulse
is applied, the piezoelectric element emits an ultrasonic wave into
the media to which it is coupled at the frequencies contained in
the excitation pulse. Conversely, when an ultrasonic wave strikes
the piezoelectric element, the element produces a corresponding
voltage across its electrodes. Typically, the front of the element
is covered with an acoustic matching layer that improves the
coupling with the media in which the ultrasonic waves propagate. In
addition, a backing material is disposed to the rear of the
piezoelectric element to absorb ultrasonic waves that emerge from
the back side of the element so that they do not interfere. A
number of such ultrasonic transducer constructions are disclosed in
U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and
4,569,231, all of which are assigned to the instant assignee.
When used for ultrasound imaging, the transducer typically has a
number of piezoelectric elements arranged in an array and driven
with separate voltages (apodizing). By controlling the time delay
(or phase) and amplitude of the applied voltages, the ultrasonic
waves produced by the piezoelectric elements (transmission mode)
combine to produce a net ultrasonic wave focused at a selected
point. By controlling the time delay and amplitude of the applied
voltages, this focal point can be moved in a plane to scan the
subject.
The same principles apply when the transducer is employed to
receive the reflected sound (receiver mode). That is, the voltages
produced at the transducer elements in the array are summed
together such that the net signal is indicative of the sound
reflected from a single focal point in the subject. As with the
transmission mode, this focused reception of the ultrasonic energy
is achieved by imparting separate time delays (and/or phase shifts)
and gains to the signal from each transducer array element.
This form of ultrasonic imaging is referred to as "phased array
sector scanning", or "PASS". Such a scan is comprised of a series
of measurements in which the steered ultrasonic wave is
transmitted, the system switches to receive mode after a short time
interval, and the reflected ultrasonic wave is received and stored.
Typically, the transmission and reception are steered in the same
direction (.theta.) during each measurement to acquire data from a
series of points along a scan line. The receiver is dynamically
focused at a succession of ranges (R) along the scan line as the
reflected ultrasonic waves are received. The time required to
conduct the entire scan is a function of the time required to make
each measurement and the number of measurements required to cover
the entire region of interest at the desired resolution and
signal-to-noise ratio. For example, a total of 128 scan lines may
be acquired over a 90 degree sector, with the steering of each scan
line being advanced in increments of 0.70.degree.. A number of such
ultrasonic imaging systems are disclosed in U.S. Pat. Nos.
4,155,258; 4,155,260; 4,154,113; 4,155,259; 4,180,790; 4,470,303;
4,662,223; 4,669,314 and 4,809,184, all of which are assigned to
the instant assignee.
The proper operation of a PASS imaging system presumes that the
speed of sound in the media through which the ultrasonic pulses are
conveyed is relatively uniform. In medical applications this
presumption is usually correct, once the sound propagates through
the body wall and enters the internal organs. Quite often, however,
irregularities in the body wall itself can produce aberrations.
Such aberrations may, for example, slow the sound emanating from
certain elements in the array such that they do not have the
desired phase when summed with the other signals at the desired
focal point. As a result, the delayed ultrasonic signal from these
elements may actually subtract from the echo signal produced by a
reflector at the focal point and thereby introduce an error, or
artifact, in the reconstructed image. In addition, the delay
introduced by the aberration is typically not constant as a
function of beam steering angle (.theta.) since the body wall has a
finite thickness. Propagation of sound through this inhomogeneous
layer of finite thickness at different angles produces different
aberrations as a function of the details of the body wall
irregularities. Also, in clinical ultrasound applications the body
wall moves due to patient breathing and other patient motion, so
that aberrations change from scan-to-scan and require recalculation
of corrections on a real-time basis.
U.S. Pat. Nos. 4,835,689 and 4,989,143, assigned to the instant
assignee, disclose a method and system for correcting the phase of
the separate signals produced by the transducer array elements to
account for such aberrations. In this prior method, the separate
signals produced by each array element are examined and phase
corrections for each element are calculated. This approach requires
access to the signal produced by each transducer element and
requires separate calculation circuitry for each element. In a
typical 64 element system, this results in considerable
hardware.
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for
correcting phase errors in a PASS imaging system caused by
aberrations in the sound media, and particularly, to the
calculation of correction phases for each transducer element using
acquired beam data. More specifically, the present invention
includes a multi-element ultrasonic transducer; a transmitter which
applies pulses to the separate transducer elements delayed by
amounts necessary to steer an ultrasonic beam in a desired
direction (.theta.); a receiver responsive to each transducer
element and which provides a separate delay to the echo signal
produced by each element to form a plurality of receive beams, one
of which is steered in the desired direction (.theta.); and an
aberration correction processor responsive to the receiver and
operable to calculate the cross correlation of the simultaneously
formed receive beams in order to measure the complex error
(A.theta., .DELTA..phi..theta.) between an echo signal from the
steering direction .theta. and the echo signals from other
reference steering angles. The aberration correction processor is
also operable to Fourier transform the set of measured errors
(A.theta., .DELTA..phi..theta.) to produce a set of correction
phase .DELTA..phi..sub.k changes are applied to correct the timing
of the pulses applied to the respective transducer elements by the
transmitter and which are applied to correct the delays imposed by
the receiver on the echo signals produced by the respective
transducer elements. As a result, when a subsequent beam is
transmitted and received along the steering direction .theta.,
artifacts caused by aberrations in the beam path are reduced.
A general object of the invention is to correct for aberrations in
the sound transmission media by using the receive beam data. Two
simultaneously-produced receive beams should have the same phase at
equal ranges if the media through which they travel is acoustically
homogeneous. To the extent they do not, a phase difference can be
measured by cross correlating the two beams. By producing a set of
such simultaneous beams and measuring the difference of each with
respect to the receive beam at the desired steering angle
(.theta.), a set of phase correction values can be produced for the
receiver and the transmitter. These correction values are used in
the subsequent acquisition of a complete transmit/receive beam at
the desired steering angle (.theta.). The process is repeated for
other steering angles until the sector scan is completed.
Another object of the invention is to reduce complexity of the
circuitry required to correct for aberrations in the sound
transmission media. The receive beam data are processed by a single
correlation and Fourier transform processor to produce the separate
phase correction values .DELTA..phi..sub.k. This is in contrast to
prior systems which employ separate circuitry for each receiver
channel.
Yet another object of the invention is to acquire measurements from
which phase corrections can be made even when there is movement of
the reflectors in the subject under examination. This is
accomplished by producing simultaneous receive beams which "see"
the same reflectors and which, therefore, facilitate accurate
measurement of the phase difference caused by aberrations in the
sound transmission media.
The foregoing and other objects and advantages of the invention
will appear from the following description. In the description,
reference is made to the accompanying drawings which form a part
hereof, and in which there is shown by way of illustration a
preferred embodiment of the invention. Such embodiment does not
necessarily represent the full scope of the invention, however, and
reference is made therefore to the claims herein for interpreting
the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an ultrasonic imaging system which
employs the present invention;
FIG. 2 is a block diagram of a transmitter which forms part of the
system of FIG. 1;
FIGS. 2A and 2B are graphical illustrations of the signal in any of
the channels of transmitter 50 of FIG. 2;
FIG. 3 is a block diagram of a receiver which forms part of the
system of FIG. 1;
FIG. 4 is a block diagram of a display system which forms part of
the system of FIG. 1;
FIG. 5 is a block diagram of a receiver channel which forms part of
the receiver of FIG. 3;
FIG. 6 is an electrical block diagram of the midprocessor which
forms part of the receiver of FIG. 3;
FIG. 7 is a schematic representation of the ultrasonic transducer
used in the system of FIG. 1 which illustrates the effect of
aberrations in the sound transmission media;
FIG. 8 is a schematic representation of the ultrasonic transducer
which illustrates the production of two simultaneous receive
beams;
FIG. 9 is a schematic representation of the ultrasonic transducer
which illustrates the orientation of the reference beams used to
practice the present invention;
FIGS. 10A and 10B are graphic representations of error measurements
made at the reference beam angles according to the present
invention;
FIG. 11 is a flow chart of the scan program executed by the digital
controller of FIG. 1;
FIG. 12 is a flow chart of the program executed by the
mid-processor of FIG. 6 to carry out the present invention;
FIG. 13 is a schematic representation of data structure in the
mid-processor of FIG. 6 produced while practicing the present
invention; and
FIGS. 14A-14E are graphical illustrations of the signal at various
points in the receiver channel of FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring particularly to FIG. 1, a vibratory energy imaging system
includes a transducer array 11 comprised of a plurality of
separately driven elements 12 which each produce a burst of
vibratory energy, such as ultrasonic energy, when energized by a
pulse produced by a transmitter 13. The vibratory energy reflected
back to transducer array 11 from the subject under study is
converted to an electrical signal by each transducer element 12 and
applied separately to a receiver 14 through a set of switches 15.
Transmitter 13, receiver 14 and switches 15 are operated under
control of a digital controller 16 responsive to commands by a
human operator. A complete scan is performed by acquiring a series
of echoes in which switches 15 are set to their transmit position,
transmitter 13 is gated on momentarily to energize each transducer
element 12, switches 15 are then set to their receive position, and
the subsequent echo signals produced by each transducer element 12
are applied to receiver 14. The separate echo signals from each
transducer element 12 are combined in receiver 14 to produce a
single echo signal which is employed to produce a line in an image
on a display system 17.
Transmitter 13 drives transducer array 11 such that the vibratory
energy produced, e.g., ultrasonic energy, is directed, or steered,
in a beam. A B-scan can therefore be performed by moving this beam
through a set of angles from point-to-point rather than physically
moving transducer array 11. To accomplish this, transmitter 13
imparts a time delay (T.sub.i) to the respective pulses 20 that are
applied to successive transducer elements 12. If the time delay is
zero (T.sub.i =0), all the transducer elements 12 are energized
simultaneously and the resulting ultrasonic beam is directed along
an axis 21 normal to the transducer face and originating from the
center of transducer array 11. As the time delay (T.sub.i) is
increased, as illustrated in FIG. 1, the ultrasonic beam is
directed downward from central axis 21 by an angle .theta.. The
relationship between the time delay increment T.sub.i added
successively to each i.sub.th signal from one end of the transducer
array (i=1) to the other end (i=N) is given by the following
relationship:
where d=equal spacing between centers of adjacent transducer
elements 12,
c=velocity of sound in the object under study,
Rr=range at which transmit beam is to be focused, and
T.sub.O =delay offset which insures that all calculated values
(T.sub.i) are positive values.
The first term in this expression steers the beam in the desired
angle .theta., and the second is employed when the transmitted beam
is to be focused at a fixed range. A sector scan is performed by
progressively changing time delays t.sub.i in successive
excitations. The angle .theta. is thus changed in increments to
steer the transmitted beam in a succession of directions. When the
direction of the beam is above central axis 21, the timing of
pulses 20 is reversed, but the formula of equation (1) still
applies.
Referring still to FIG. 1, the echo signals produced by each burst
of ultrasonic energy emanate from reflecting objects located at
successive positions along the ultrasonic beam. These are sensed
separately by each segment 12 of transducer array 11 and a sample
of the magnitude of the echo signal at a particular point in time
represents the amount of reflection occurring at a specific range
(R). Due to differences in the propagation paths between a focal
point P and each transducer element 12, however, these echo signals
will not occur simultaneously and their amplitudes will not be
equal. The function of receiver 14 is to amplify and demodulate
these separate echo signals, impart the proper time delay to each
and sum them together to provide a single echo signal which
accurately indicates the total ultrasonic energy reflected from
each focal point P located at range R along the ultrasonic beam
oriented at the angle .theta..
To simultaneously sum the electrical signals produced by the echoes
from each transducer element 12, time delays are introduced into
each separate transducer element channel of receiver 14. In the
case of linear array 11, the delay introduced in each channel may
be divided into two components; one component is referred to as a
beam steering time delay, and the other component is referred to as
a beam focusing time delay. The beam steering and beam focusing
time delays for reception are precisely the same delays (T.sub.i)
as the transmission delays described above. However, the focusing
time delay component introduced into each receiver channel is
continuously changing during reception of the echo to provide
dynamic focusing of the received beam at the range R from which the
echo signal emanates. This dynamic focusing delay component is as
follows:
where R=range of the focal point P from the center of array 11,
C=velocity of sound in the object under study, and
T.sub.k =desired time delay associated with the echo signal from
the kth element to coherently sum it with the other echo
signals.
Under direction of digital controller 16, receiver 14 provides
delays during the scan such that steering of receiver 14 tracks
with the direction (.theta.) of the beam steered by transmitter 13
and it samples the echo signals at a succession of ranges (R) and
provides the proper delays to dynamically focus at points P along
the beam. Thus, each emission of an ultrasonic pulse results in
reception of a series of echo signal samples which represent the
amount of reflected sound from a corresponding series of points P
located along the ultrasonic beam. Receiver 14 is able to change
its delays for each echo signal sample to dynamically focus on the
reflectors which produce the signal sample. The stream of focused
and steered echo signal samples which are produced by the receiver
is referred to in the art as the "received beam".
Display system 17 receives the series of data samples produced by
receiver 14 and converts the data to a form producing the desired
image. For example, if an A-scan is desired, the magnitude of the
series of data points is merely graphed as a function of time. If a
B-scan is desired, each data point in the series is used to control
brightness of a pixel in the image, and a scan comprised of a
series of measurements at successive steering angles (.theta.) is
performed to provide the data necessary for display.
It can be appreciated that the time delays produced in accordance
with equation (1) to provide the desired steering and focusing
during both the transmit mode and receive mode presume that the
sound travels at a uniform velocity c throughout the sound
propagating media. In clinical applications this is usually not the
case. Instead, as illustrated in FIG. 7, ultrasonic transducer 11
is typically placed on the patient's skin 25 and sound emanating
therefrom passes through one or more layers of tissue which have
different sound propagating properties. A boundary 26 between such
layers has an irregular shape. As a result, for example, when beam
samples are being acquired from point P at steering angle .theta.,
sound conveyed between point P and two separate array elements
12.sub.4 and 12.sub.5 is propagated quite differently due to the
irregularity of boundary 26. This is illustrated by the respective
dashed lines 28 and 29 which reveal that the ultrasound path length
in tissue layer 30 is longer for transducer element 12.sub.5 than
for transducer element 12.sub.4. This difference in path length in
tissue layer 30 to the two transducer elements causes the phase
aberration. The same tissue layer may or may not affect the phase
of sound associated with others of transducer elements 12, and the
same tissue layer may or may not affect the phase of the sound
associated with the same elements 12.sub.4 and 12.sub.5 at
different steering angles .theta.. This is illustrated by sound
paths 31 and 32 to a point P', which paths have very similar
lengths in tissue layer 30.
The corrections for phase errors caused by aberrations in the sound
propagating media are different for each transducer element 12 and
for each steering angle .theta. acquired during the scan. According
to the present invention, the phase corrections .DELTA..phi..sub.k
required by the signals associated with each transducer element 12
in order to offset the errors caused by aberrations in the sound
media are calculated, and those corrections are applied to the time
delays T.sub.i produced by transmitter 13 and imposed by receiver
14, shown in FIG. 1.
Referring to FIG. 2 in conjunction with FIG. 1, transmitter 13
includes a set of channel pulse code memories indicated
collectively as memories 50. In the preferred embodiment, there are
64 separate transducer elements 12, and therefore, there are 64
separate channel pulse code memories 50. Each pulse code memory 50
is typically a 1-bit by 512-bit memory which stores a bit pattern
51 that determines the frequency of ultrasonic pulse 52 to be
produced. In the preferred embodiment, this bit pattern is read out
of each pulse code memory 50 by a 40 MHz master clock and applied
to a driver 53 which amplifies the signal to a power level suitable
for driving transducer 11. Transducer elements 12 to which these
ultrasonic pulses 52 are applied respond by producing ultrasonic
energy. If all 512 bits are used, then a pulse of bandwidth as
narrow as 40 kHz centered on the carrier frequency will be
emitted.
As indicated above, to steer the transmitted beam of the ultrasonic
energy in the desired direction (.theta.), pulses 52 for each of
the N channels, such as shown in FIG. 2B, must be delayed by the
proper amount. These delays are provided by a transmit control 54
which receives four control signals (START, MASTER CLOCK, R.sub.T
and .theta.) from digital controller 16 (FIG. 1). Using the input
control signal .theta., the fixed transmit focus R.sub.T, and the
above equation (1), transmit control 54 calculates the delay
increment T.sub.i required between successive transmit channels.
When the START control signal is received, transmit control 54
gates one of four possible phases of the 40 MHz MASTER CLOCK signal
through to the first transmit channel 50. At each successive delay
time interval (T.sub.I) thereafter, one of the phases of the 40 MHz
MASTER CLOCK signal is gated through to the next channel pulse code
memory 50 until all N=64 channels are producing their ultrasonic
pulses 52. Each transmit channel 50 is reset after its entire bit
pattern 51, such as shown in FIG. 2A, has been transmitted and
transmitter 13 then waits for the next .theta. and next START
control signals from digital controller 16. As indicated above, in
the preferred embodiment of the invention, a complete B-scan is
comprised of 128 ultrasonic pulses steered in .DELTA..theta.
increments of 0.70 degrees through a 90 degree sector centered
about the central axis 21 (FIG. 1) of the transducer 11.
For a detailed description of transmitter 13, reference is made to
commonly assigned U.S. Pat. No. 5,014,712, issued Jan. 28, 1991 and
entitled "Coded Excitation for Transmission Dynamic Focusing of
Vibratory Energy Beam", incorporated herein by reference.
Referring particularly to FIG. 3 in conjunction with FIG. 1,
receiver 14 is comprised of three sections: a time-gain control
section 100, a receive beam forming section 101, and a mid
processor 102. Time-gain control section 100 includes an amplifier
105 for each of the N=64 receiver channels and a time gain control
circuit 106. The input of each amplifier 105 is connected to a
respective one of transducer elements 12 to receive and amplify the
echo signal which it receives. The amount of amplification provided
by amplifiers 105 is controlled through a control line 107 that is
driven by time-gain control circuit 106. As is well known in the
art, as the range of the echo signal increases, its amplitude is
diminished. As a result, unless the echo signal emanating from more
distant reflectors is amplified more than the echo signal from
nearby reflectors, the brightness of the image diminishes rapidly
as a function of range (R). This amplification is controlled by the
operator who manually sets eight (typically) TGC linear
potentiometers 108 to values which provide a relatively uniform
brightness over the entire range of the sector scan. The time
interval over which the echo signal is acquired determines the
range from which it emanates, and this time interval is divided
into eight segments by TGC control circuit 106. The settings of the
eight potentiometers are employed to set the gains of amplifiers
105 during each of the eight respective time intervals so that the
echo signal is amplified in ever increasing amounts over the echo
signal acquisition time interval.
The receive beam forming section 101 of receiver 14 includes N=64
separate receiver channels 110. As will be explained in more detail
below, each receiver channel 110 receives the analog echo signal
from one of TGC amplifiers 105 at an input 111, and it produces a
stream of digitized output values on an I bus 112 and a Q bus 113.
Each of these I and Q values represents a sample of the echo signal
envelope at a specific range (R). These samples have been delayed
in the manner described above such that when they are summed at
summing points 114 and 115 with the I and Q samples from each of
the other receiver channels 110, they indicate the magnitude and
phase of the echo signal reflected from a point P located at range
R on the steered beam (.theta.). In the preferred embodiment, each
echo signal is sampled at equal intervals of about 150 micrometers
over the entire range of the scan line (typically 40 to 200
millimeters).
For a more detailed description of receiver 14, reference is made
to U.S. Pat. No. 4,983,970 which issued on Jan. 8, 1991 and is
entitled "Method and Apparatus for Digital Phase Array Imaging",
and which is incorporated herein by reference.
Referring still to FIG. 3, mid processor section 102 receives the
receive beam samples from summing points 114 and 115. The I and Q
values of each beam sample are 16-bit digital numbers representing
the in-phase and quadrature components of the magnitude of
reflected sound from a point (R,.theta.). Mid processor 102 can
perform a variety of calculations on these beam samples, where
choice is determined by the type of image to be reconstructed. For
example, if a conventional magnitude image is to be produced, a
detection processor 120 is implemented in which a digital magnitude
M is calculated from each receive beam sample and produced at
output at 121 according to ##EQU1##
The present invention is implemented in large part by an aberration
correction processor 122 contained in mid processor 102 and
described in detail below. Aberration correction processor 122
receives the I and Q components of two simultaneously produced beam
samples from beam forming section 101 and calculates phase
correction values .DELTA..phi..sub.k which are produced at mid
processor output 123. These phase correction values
.DELTA..phi..sub.k are applied to transmitter 13 of FIG. 1 as
described above, and they are applied to the separate channels 110
of the receiver beam forming section, as described in detail below.
During a scan, at each steering angle .theta. data are first
acquired and used by aberration correction processor 122 to produce
the phase correction values .DELTA..phi..sub.k. On the next
acquisition along this beam, either in the next image frame or on a
subsequent firing in the same image frame, image data are acquired
with the phase corrections in place. The image data are used by
detection processor 120 to produce the data for display system 17
of FIG. 1.
Referring particularly to FIGS. 1 and 4, receiver 14 generates a
stream of 8-bit digital numbers at its output 121, which is applied
to the input of display system 17. This "scan data" is stored in a
memory 150 as an array, with the rows of scan data array 150
corresponding with the respective beam angles (.theta.) that are
acquired, and the columns of scan data array 150 corresponding with
the respective ranges (R) at which samples are acquired along each
beam. The R and .theta. control signals 151 and 152 from receiver
14 indicate where each input value is to be stored in array 150,
and a memory control circuit 153 writes that value to the proper
memory location in array 150. The scan can be continuously repeated
and the flow of values from receiver 14 will continuously update
scan data array 150.
Referring still to FIG. 4, the scan data in array 150 are read by a
digital scan converter 154 and converted to a form producing the
desired image. If a conventional B-scan image is being produced,
for example, the magnitude values M(R,.theta.) stored in scan data
array 150 are converted to magnitude values M(x,y) which indicate
magnitudes at pixel locations (x,y) in the image. Such a polar
coordinate to Cartesian coordinate conversion of the ultrasonic
image data is described, for example, in an article by Steven C.
Leavitt et al. in Hewlett-Packard Journal, Oct., 1983, pp. 30-33,
entitled "A Scan Conversion Algorithm for Displaying Ultrasound
Images".
Regardless of the particular conversion made by digital scan
converter 154, the resulting image data are written to a memory 155
which stores a two-dimensional array of converted scan data. A
memory control 156 provides dual-port access to memory 155 such
that the digital scan converter 154 can continuously update the
values therein with fresh data while a display processor 157 reads
the updated data. Display processor 157 is responsive to operator
commands received from a control panel 158 to perform conventional
image processing functions on the converted scan data in memory
155. For example, the range of brightness levels indicated by the
converted scan data in memory 155 may far exceed the brightness
range of display device 160. Indeed, the brightness resolution of
the converted scan data in memory 155 may far exceed the brightness
resolution of the human eye, and manually operable controls are
typically provided which enable the operator to select a window of
brightness values over which maximum image contrast is to be
achieved. The display processor reads the converted scan data from
memory 155, provides the desired image enhancement, and writes the
enhanced brightness values to a display memory 161.
Display memory 161 is shared with a display controller circuit 162
through a memory control circuit 163, and the brightness values
therein are mapped to control brightness of the corresponding
pixels in display 160. Display controller 162 is a commercially
available integrated circuit designed to operate the particular
type of display 160 used. For example, display 160 may be a CRT
(cathode ray tube), in which case display controller 162 is a CRT
controller chip which provides the required sync pulses for the
horizontal and vertical sweep circuits and maps the display data to
the CRT at the appropriate time during the sweep.
It should be apparent to those skilled in the art that display
system 17 may take one of many forms depending on the capability
and flexibility of the particular ultrasound system. In the
preferred embodiment described above, programmed microprocessors
are employed to implement the digital scan converter and display
processor functions, and the resulting display system is,
therefore, very flexible and powerful.
As indicated above with reference to FIG. 3, beam forming section
101 of receiver 14 is comprised of a set of receiver channels
110--one for each element 12 of transducer 11 (FIG. 1). Referring
particularly to FIG. 5, each receiver channel is responsive to a
START command, a 40 MHz master clock, a range signal (R) and a beam
angle signal (.theta.) from digital controller 16 (FIG. 1) to
perform the digital beam forming functions. These include: sampling
the analog input signal in an analog-to-digital converter 200,
demodulating the sampled signal in a demodulator 201; filtering out
the high frequency sum signals produced by demodulator 201 with low
pass filters 202; reducing the data rate in decimators 203; and
time delaying and phase adjusting the resulting digital data stream
in delay FIFOs (i.e., first-in/first-out memories) 204 and phase
rotator 205. All of these elements are controlled by a receive
channel control 206 which produces the required clock and control
signals in response to commands from digital controller 16 (FIG.
1). In addition, an aberration correction phase .DELTA..phi..sub.k
is also provided to receive channel control 206 and added to the
normal delay T.sub.k imposed on the echo signal. In the preferred
embodiment all of these elements are contained on a single
integrated circuit.
Referring still to FIG. 5, analog-to-digital converter 200 samples
the analog input signal, indicated graphically by waveform 210 in
FIG. 14A, at regular intervals determined by the leading edge of a
delayed sample clock signal from receive channel control 206. In
the preferred embodiment, the sample clock signal is a 40 MHz clock
signal to enable use of ultrasonic frequencies up to 20 MHz without
violating the Nyquist sampling criteria. When a 5 MHz ultrasonic
carrier frequency is employed, for example, it is sampled eight
times per carrier cycle and a 10-bit digital sample is produced at
the output of the analog-to-digital converter at a 40 MHz rate.
These samples are supplied to demodulator 201 which mixes each
sample with both a reference in-phase with the transmitted
ultrasonic carrier, and with a reference in quadrature with the
transmitted ultrasonic carrier. The demodulator reference signals
are produced from stored SINE and COSINE tables that are read out
of their respective ROM memories by a 40 MHz reference clock signal
from receive channel control 206. The SINE value is digitally
multiplied by the sampled input signal to produce a demodulated,
in-phase value (I) supplied to low pass filter 202, and the COSINE
value is digitally multiplied by the same sampled input signal to
produce a demodulated, quadrature phase value Q output signal to a
separate low pass filter 202. Low pass filters 202 are finite
impulse response filters tuned to pass the difference frequencies
supplied by demodulator 201, but block the higher, sum frequencies.
As shown by waveform 215 in the graph of FIG. 14B, the output
signal of each low pass filter is, therefore, a 40 MHz stream of
digital values which indicate the magnitude of the I or Q component
of the echo signal envelope.
For a detailed description of an analog-to-digital converter,
demodulator, and a low pass filter circuit, reference is made to
commonly assigned U.S. Pat. No. 4,839,652 which issued Jun. 13,
1989 and is entitled "Method and Apparatus for High Speed Digital
Phased Array Coherent Imaging System".
Referring still to FIG. 5, the rate at which the demodulated I and
Q components of the echo signal are sampled is reduced by
decimators 203. The 12-bit digital samples are supplied to the
decimators at a 40 MHz rate which is unnecessarily high from an
accuracy standpoint, and which is a difficult data rate to maintain
throughout the system. Accordingly, decimators 203 select every
eighth digital sample to reduce the data rate down to a 5 MHz rate.
This corresponds to the frequency of a baseband clock signal
produced by receive channel control 206 and employed to operate the
remaining elements in the receiver channel. The I and Q output
signals of decimators 203 are thus digitized samples 219 of the
echo signal envelope indicated by dashed line 220 in the graph of
FIG. 14C. The decimation ratio and the baseband clock frequency can
be changed to values other than 8:1 and 5 MHz.
The echo signal envelope represented by the demodulated and
decimated digital samples is then delayed by delay FIFOs 204 and
phase rotator 205 to provide the desired beam steering and beam
focusing. These delays are in addition to the coarse delays
provided by the timing of the delayed sample clock signal applied
to analog-to-digital converter 200 as described above. That is, the
total delay provided by receiver channel 110 is the sum of the
delays provided by the delayed sample clock signal supplied to
analog-to-digital converter 200, delay FIFOs 204 and phase rotator
205. This total delay is equal to the calculated value T.sub.i in
accordance with equation (1), plus the delay represented by the
aberration phase correction .DELTA..phi..sub.k. The delay FIFOs 204
are memory devices into which the successive digital sample values
are written as they are produced by the decimators 203 at a rate of
5 MHz. These stored values are written into successive memory
addresses and then read from the memory device and supplied to
phase rotator 205. The amount of delay, illustrated graphically in
FIG. 14D, is determined by the difference between the memory
location from which the digital sample is currently being supplied
and the memory location into which the currently received digital
sample is being stored. The 5 MHz baseband clock signal establishes
200 nanosecond intervals between stored digital samples and FIFOs
204 can, therefore, provide a time delay measured in 200 nanosecond
increments up to their maximum of 25.6 microseconds.
Phase rotator 205 enables the digitized representation of the echo
signal to be delayed by amounts less than the 200 nanosecond
resolution of delay FIFOs 204. The I and Q digital samples supplied
to phase rotator 205 may be represented, as shown in FIG. 14E, by a
phasor 221 and the rotated I and Q digital samples produced by
phase rotator 205 may be represented by a phasor 222. The
magnitudes of the phasors (i.e. the vector sum of the I and Q
components of each) are not changed, but the I and Q values are
changed with respect to one another such that the output phasor 222
is rotated by an amount .DELTA..phi. from the input phasor 221. The
phase can be either advanced (+.DELTA..phi.) or delayed
(-.DELTA..phi.)in response to a phase control signal received on a
bus from receive channel control 206. For a detailed description of
phase rotator 205, reference is made to commonly assigned U.S. Pat.
No. 4,896,287 which issued on Jan. 23, 1990, entitled "Cordic
Complex Multiplier", and incorporated herein by reference.
Referring to FIG. 6, the I and Q output signals of each channel
receiver 110 are supplied to first and second phase rotators 250
and 252. Phase rotators 250 and 252 may be included as part of each
receiver channel 110. Phase rotators 250 and 252 form two receive
beams as described in U.S. Pat. No. 4,886,069 which issued on Dec.
12, 1989 and is hereby incorporated by reference, and is entitled
"Method Of, And Apparatus For, Obtaining A Plurality Of Different
Return Energy Imaging Beams Responsive To A Single Excitation
Event." Each of phase rotators 250 and 252 is identical to phase
rotator 205 (FIG. 5) described above, and receives phase shift
commands .DELTA..theta..sub.1 and .DELTA..theta..sub.2 respectively
from the digital controller 16 (FIG. 1). The I and Q output signals
from phase rotator 250 are summed with the corresponding output
signals of all the other channels 110 at summing points 262 and 263
to form a first receive beam that produces a stream of 16-bit beam
data S(R,.theta.+.DELTA..theta..sub.1) at an input latch 264.
Similarly, the I and Q output signals from phase rotator 252 are
summed with corresponding channel output signals at summing points
276 and 268 to form a second receive beam that produces a stream of
16-bit. beam sample data S(R,.theta.+.DELTA..theta..sub.2) at an
input latch 269. For a more detailed description of how the I and Q
output signals of each receiver channel 110 are summed together to
form the two beam signals, see U.S. Pat. No. 4,983,970 which issued
on Jan. 8, 1991 and is entitled " Method and Apparatus For Digital
Phased Array Imaging", which patent is hereby incorporated by
reference. As illustrated in FIG. 8, the receiver thus forms two
simultaneous receive beams located about the steering angle .theta.
by an amount determined by the beam offsets .DELTA..theta..sub.1
and .DELTA..theta..sub.2. As will become apparent from the
description below, use of these simultaneously-produced receive
beams to make the phase measurements required to practice the
present invention is an important aspect of the invention.
Referring still to FIG. 6, mid processor 102 (FIG. 3) is structured
around a 16-bit processor 280 which drives a 16-bit data bus 281.
Processor 280 is programmed to read the receive beam samples from
input latches 264 and 269 as they become available, and to load
them in the proper location in an S(R,.theta.) array 283 in memory
282. As explained in more detail below, when the errors in "beam
space" have been adequately sampled, the sample values stored in
array 283 are employed to calculate a set of beam space errors
(A.theta., .DELTA..phi..theta.) stored in memory array 284.
These values are used to define a function F(.theta.) which relates
measured error to beam angle .theta.. This function F(.theta.) is
Fourier transformed to produce 128 "element space" phase correction
values .DELTA..phi..sub.k which are stored in a memory array 285
and supplied to a latch 286 for application to transmitter 13 (FIG.
1) and receiver beam former 101 (FIG. 3) as described above.
After the phase corrections are provided for a particular beam
angle .theta., image data subsequently acquired from that angle
.theta. and the samples S(R,.theta.) appear at input latch 264.
Processor 280 is programmed to read the I and Q components of these
sample values, compute the magnitude values M(R,.theta.) as
described above, and supply them to shared memory 150 in display
system 17 (FIG. 4). A scan is thus comprised of a series of
measurements at each beam angle (.theta.) which enable phase
adjustments to be made to the transmitter and receiver, and then
the image data for that beam angle (.theta.) are acquired. This is
repeated for each of the 128 beam angles (.theta.) in the scan.
To perform the scan according to the present invention the program
executed by digital controller 16 (FIG. 1) must be adapted to
operate in cooperation with the program executed by processor 280
in the receiver (FIG. 6). Before explaining these programs,
however, the manner and order in which the phase measurements are
made will be described.
Referring particularly to FIG. 9, for each beam angle .theta. to be
produced during the scan a set of correlation measurements are made
by computing the cross-correlation function between the complex,
baseband outputs of the two simultaneous receive beams over a
finite duration. The signals used for correlation processing
emanate from a random, or near-random, collection of scatterers
distributed over range. Typically, signals received from the focal
zone of the transmitter are used for this purpose. From these
correlation measurements the aberration correction phases
.DELTA..phi..sub.k are produced. In the preferred embodiment of the
invention, the correlation measurements are made at seventeen
equally spaced reference angles .theta..sub.1 -.theta..sub.17
throughout the 90.degree. sector to be scanned. Two beams are
produced simultaneously, one at angle .theta..sub.0, and the other
at one of the reference angles .theta..sub.1 -.theta..sub.17 as
described above, and they are cross correlated over a range segment
to calculate both the relative amplitudes A.theta. and phases
.DELTA..phi..sub..theta. between the beams. In addition, the
autocorrelation of the beam at angle .theta..sub.0 is computed for
proper normalization.
Referring particularly to FIGS. 10a and 10b, after the seventeen
correlation measurements are made, a complex function, F(.theta.)
can be defined by passing a smooth curve through the measured
correlation values (A.theta., .DELTA..phi..theta.), where the real
part of each complex value equals A.theta. cos .DELTA..phi..theta.,
and the imaginary part equals A.theta. sin .DELTA..phi..theta..
This function F(.theta.) is then sampled at 128 points spaced
equally in increments of sin.theta. along the entire range
-45.degree.<.theta.<+45.degree.. These 128 complex samples
are fast Fourier transformed to produce 128 phase corrections
.DELTA..phi..sub.k. These 128 phase corrections are applied to the
128 channels in the transmitter and receiver which correspond to
the respective 128 transducer elements 12 (FIG. 1). Thus the phase
errors are measured in "beam space" and Fourier transformed into
"element space" where they can effectively be applied to correct
the transmitter and receiver.
Referring again to FIG. 9, while the phase measurements
.DELTA..phi..sub..theta. determine the phase error between the
desired beam (.theta..sub.o) and each of the seventeen reference
angles .theta..sub.1 through .theta..sub.17, in fact, these
measurements are obtained indirectly in the preferred embodiment.
More specifically, the phase error .DELTA..phi..theta. is measured
between the desired beam (.theta..sub.0) and the nearest reference
angle (.theta..sub.7 in FIG. 9), and the phase error
.DELTA..phi..theta. is then measured between adjacent pairs of
reference angles (.theta..sub.7 -.theta..sub.8, -.theta..sub.9,
.theta..sub.9 -.theta..sub.10, etc.). The phase error
.DELTA..phi..theta. between the desired beam angle (.theta..sub.0)
and each of the reference angles .theta..sub.1 through
.theta..sub.17 is then calculated by adding together the separate,
"incremental" measurements. In this manner the phase errors
throughout the 90.degree. sector can be measured using two
simultaneous receive beams that are separated from one another by
less than six degrees.
Referring particularly to FIG. 11 in conjunction with FIG. 1, to
carry out the preferred embodiment of the present invention,
digital controller 16 executes a scan program entered at step 300.
Data structures including a phase correction flag and a reference
angle counter are initialized at process step 301, and then a loop
is entered in which the phase measurements for a desired beam angle
(.theta.) are made. As indicated at process step 302, the sample
angle of each phase measurement is calculated such that the
transmit beam is directed toward the reference angle closest to the
desired beam angle (.theta.), and the two receive beams are
directed at two of the reference beam directions. Referring to FIG.
9, for example, if the phase error between reference angles
.theta..sub.7 and .theta..sub.8 is to be measured, the transmit
beam is directed at the closest reference angle (.theta..sub.0
=.theta..sub.7) and the receive beams are directed at the angles
.theta..sub.7 and .theta..sub.8. The beam is transmitted, as
indicated at process step 303, and two receive beams are received
at process step 304. The phase shift commands .DELTA..theta..sub.1
and .DELTA..theta..sub.2 provided to the receiver during the
reception of the echo signal are set to direct the two receive
beams at the pair of reference angles .theta..sub.1 -.theta..sub.17
being measured, and the timing of the receive beam former is set to
a beam directed midway between these two reference angles. This
process continues as described above until all seventeen of the
correlation measurements have been made as determined at decision
step 305.
After correlation data have been acquired, a phase correction
message is supplied to the mid processor of receiver 14 to indicate
that the phase corrections should be calculated as indicated at
process step 306. Digital controller 16 then waits at decision step
307 for a return message which indicates that the calculations have
been made and the aberration correction phases .DELTA..phi..sub.k
have been applied to transmitter 13 and receiver 14 as described
above. Then, transmitter 13 is enabled at process step 308 of FIG.
11 to generate an ultrasonic pulse steered at the beam angle
(.theta.) and receiver 14 is enabled at process step 309 to receive
the resulting echo signal. The series of received beam samples are
provided to display system 17 at process step 310 to produce one
line, or beam, of data on the display screen. The scan program
loops back to produce another beam in the image until all 128 beams
have been produced as determined at decision step 311.
It should be apparent to those skilled in the art that a number of
variations are possible in the manner in which the scan is
performed. For example, it is not always necessary to remeasure the
incremental phase differences for each beam angle (.theta.).
Instead, independent measurements can be made at each of the
reference angles to characterize the phase errors
.DELTA..phi..sub.k as a function of steering beam angle .theta..
For beam angles between these reference angles, interpolation may
be employed using the phase errors .DELTA..phi..sub.k from the two
adjacent reference angles. Also, measurements need not be made on
every frame. A measurement may be applied to several frames of data
before a new measurement is made. The rate at which the full set of
phase measurements are made may be operator selectable so that the
physician can trade off image quality and total scan time as the
examination is being performed.
Referring particularly to FIG. 12, processor 280 (FIG. 6) in mid
processor 102 (FIG. 3) is programmed to receive the sample beam
data and either use it to calculate aberration phase corrections
.DELTA..phi..sub.k or produce a beam of image data for display
system 17 (FIG. 1). The program is entered at step 325 and data
structures such as a "corrections done" flag are initialized at
process step 326 before determining what mode the mid processor is
in. If the mid processor is in the aberration correction mode as
determined at decision step 327, it waits in accordance with
decision step 328 until receive data are available. As indicated
above, the mid processor is placed in this mode by a message from
digital controller 16 (FIG. 1).
Referring particularly to FIGS. 6 and 12, when data are available
at input latches 264 and 269, as determined at decision step 328,
the data are read and stored in S(R,.theta.) array 283 as indicated
at process step 329. When seventeen pairs of beam data have been
acquired, a message is received from digital controller 16
indicating that phase corrections are to be calculated. This is
detected at decision step 330, and at this time S(R,.theta.) array
283 stores the data illustrated in FIG. 13.
Array 283 of FIG. 6 is comprised of two two-dimensional arrays 331
and 332, shown in FIG. 13. Each row of array 331 stores the beam
samples from input latch 264 (FIG. 6) and the corresponding row in
array 332 stores the beam samples from the simultaneously received
beam. The first row of beam sample data in array 331 measures the
echo signal along the desired beam angle (.theta.) and the
corresponding first row in array 332 stores the simultaneously
acquired data from the nearest reference angle .theta..sub.x.
Subsequent rows in arrays 331 and 332 represent the
simultaneously-acquired pairs of adjacent reference beam data.
Referring again to FIG. 12, after the correlation measurement data
have been acquired the first step is to calculate the
cross-correlation of the echo signal pairs over a set of ranges as
indicated at process step 335. This is accomplished by cross
correlating corresponding rows in arrays 331 and 332 (FIG. 13) with
each other. This produces a set of seventeen correlation amplitude
and phase difference values. In the preferred embodiment, each
cross correlation is performed by multiplying a complex sample in
one beam by the complex conjugate of the corresponding complex
sample in the other beam at a set of ranges in the center of the
image, and averaging the results. Then, as indicated at process
step 336, these incremental differences are added together as
described above to provide the complex errors A.sub.1-17 and
.DELTA..phi..sub.1-17 which indicate the error in the desired beam
patterns. FIG. 10A shows the phase shift values
.DELTA..phi..sub..theta. as a function of beam angle (.theta.)
while, in FIG. 10B, the relative amplitude values A.sub..theta. are
normalized to the amplitude of the autocorrelation of the reference
beam .theta..sub.0 amplitude (i.e. .theta..sub.7 in the above
example) and are also shown as a function of beam angle
(.theta.).
Next, as indicated at process step 337 of FIG. 12, a smooth curve
is passed through the above-mentioned seventeen relative amplitude
and phase error measurements to produce a complex error function
F(.theta.), which measures the complex error (i.e. magnitude and
phase) as a function of beam angle, as shown by the dashed lines
340a and 340b in FIGS. 10A and 10B. This error function F(.theta.)
is sampled at 128 points and these samples, as indicated at process
step 341 of FIG. 12, undergo a fast Fourier transformation. This
produces 128 phase correction values .DELTA..phi..sub.k for each
transmit beam angle .theta.. These are provided, at process step
342, to transmitter 13 (FIG. 1) and receiver channels 110 (FIG. 3)
as described above. A "corrections done" message is then sent to
digital controller 16 (FIG. 1) at process step 343, and the mid
processor loops back to determine its next mode of operation.
Again, an independent set of correction values .DELTA..phi..sub.k
need not be acquired for every transmit beam angle .theta. used in
the scan. A reduced set of correction values .DELTA..phi..sub.k can
be acquired for transmit beams directed along the reference angles.
Interpolation can then be used to produce accurate estimates of the
correction values .DELTA..phi..sub.k at transmit beam angles
.theta. between these reference angles.
Referring still to FIG. 12, after the phase corrections have been
made by the mid processor, the mid processor is placed in the image
mode, which is detected at decision step 350. The mid processor
waits until beam samples are available at input latch 264 (FIG. 6),
and when they are available, as determined at decision step 351,
the detection processor operates to calculate the corresponding
magnitude M(R,.theta.), as indicated at process step 352. This
magnitude value is supplied to display system 17 (FIG. 1) at
process step 353 and a test is then made at decision step 354 to
determine if the entire beam at angle (.theta.) has been acquired.
If so, the mid processor loops back to determine its next mode of
operation; if not, the mid processor loops back to await more beam
samples before again undergoing decision step 351.
While only certain preferred features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *